1. Field of the Invention
The present invention relates to a demodulation technique in communication using a coding method in which each symbol is expressed by a plurality of bits on a time base.
2. Description of the Related Art
For example, a system in which a recovered clock is generated by use of a series of received data, and demodulated data is decimated from the series of received data by use of the recovered clock is proposed as a demodulator for generating demodulated data from Manchester-coded signals. FIG. 10 is a block diagram of an configuration example of a conventional demodulator, schematically showing a configuration of a demodulation circuit, for example, disclosed in Japanese Patent No. 2508502, entitled “Demodulation Circuit” (by Norio Numata, Takayuki Inoue, and Kenichi Sugawara). Further, FIG. 11 is a schematic diagram for explaining the operation of the conventional demodulator, and FIG. 12 is a flow chart showing the operation of the conventional demodulator.
In FIG. 10, the reference numeral 101 represents an input terminal into which a series of Manchester-coded received data are inputted; 102, a clock recovery circuit for generating a recovered clock by use of the series of received data, and outputting the recovered clock; 103, a phase correction circuit for correcting the phase of the recovered clock supplied from the clock recovery circuit 102 in order to generate a clock for decimation in demodulation, and outputting the corrected recovered clock as the clock for decimation; 104, a decimation circuit for decimation demodulated data out of the series of received data by use of the clock for decimation supplied from the phase correction circuit 103, and outputting the decimated demodulated data; and 105, an output terminal for the demodulated data.
Next, the operation will be described with reference to FIGS. 10 to 12. The series of Manchester-coded received data has an inversion of data in each symbol, and each symbol is formed of two bits. For example, when “1” is transmitted by an NRZ (Non Return to Zero) signal, the “1” is expressed by “10” in Manchester code, while when “0” is transmitted, the “0” is expressed by “01”. Therefore, in demodulation, Manchester decoding is also performed at the same time by thinning out the bit in the first half or the bit in the second half in each symbol.
First, when a series of received data are inputted to the input terminal 101, the operation starts. When a recovered clock is generated by use of the series of received data in the clock recovery circuit 102 (Step S201), the phase correction circuit 103 corrects the phase of the recovered clock outputted from the clock recovery circuit 102 so as to make the phase correspond to the bit in the first half or the bit in the second half in each symbol, and supplies the corrected recovered clock as a clock for decimation to the decimation circuit 104 in the succeeding stage (Step S202). The decimation circuit 104 decimates data from the series of received data by use of the clock for decimation supplied from the phase correction circuit 103 in the preceding stage, and outputs the data as demodulated data (Step S203). If the input of the series of received data disappears and the demodulation is completed, the operation is ended.
As described above, a conventional demodulator corrects the phase of a recovered clock generated by use of a series of received data so as to make the phase correspond to the bit in the first half or the bit in the second half in each symbol to thereby generate a clock for decimation, and demodulated data out of the series of received data by use of the clock for decimation to thereby perform demodulation.
However, in the conventional system, there has been a problem that a data decision point is apt to be mistaken by noise or interference because demodulation is performed by one-point data per symbol. Further, though it is intended to reduce the error rate by thinning out the bits in the second half in each symbol, there has been a problem that an error occurs easily when there is a distortion in a transmission waveform, or when the duty ratio of “H” and “L” in a symbol is deteriorated (for example, it is 4:6) because of a detector characteristic or a transmission path characteristic.